On the monodromy of moduli spaces of sheaves on K3 surfaces II

Abstract

Let S be a K3 surface. In part I of this paper, we constructed a representation of the group Aut D(S), of auto-equivalences of the derived category of S. We interpreted this infinite dimensional representation, as the natural action of Aut D(S) on the cohomology of all moduli spaces of stable sheaves (with primitive Mukai vectors) on S. The main result, of part I, is the precise relation of this action with the monodromy of the Hilbert schemes S^[n] of points on the surface. The proof of the above result was reduced, in part I, to the case of two monodromy operators of S^[n], associated with choices of line bundles on the surface S, of degree 2n-4 and 2n respectively. When n=1, the first sequence of monodromy operators specializes to the reflection by a -2 curve. The n=1 case of the second sequence is related to the Galois involution, of a double cover of the projective plane, branched along a sextic. We complete the proof by treating these two sequences of examples.